Biodegradable resins such as polylactic acid-based resins are finding increasing applications, including applications to packaging materials, applications to mulching films for the agricultural field, applications to the well drilling method for mining underground resources, and the like. With this trend, development of technologies to meet various applications has been demanded such as improvement in biodegradable resin degradation rate and development of a degradation trigger or degradation rate controlling technology. The rotary drilling method is a method in which the drilling is achieved with a drill, while muddy water is being circulated. In this method, a kind of filter membrane called mud cake is formed by using a fluid-loss-control agent as a finishing fluid. Thus, the wall of the well is kept stable to prevent collapse, and the friction is reduced. Meanwhile, in the hydraulic fracturing method, a fluid with which a well is filled is pressurized to a high pressure to form fractures in the vicinity of the well. Thus, the penetrability (flowability of fluid) in the vicinity of the well is improved, and the effective cross-section thorough which a resource such as oil or gas flows into the well is increased to increase the productivity of the well.
In the case of the finishing fluid, in which calcium carbonate or a granulated salt is mainly used as the fluid-loss-control agent, an acid treatment is necessary to remove the fluid-loss-control agent, and the stratum of the well is clogged by the fluid-loss-control agent to cause production failure. In addition, the fluid used in the hydraulic fracturing method is also called a fracturing fluid, and viscous fluids such as gasoline gel had been used in the past. With the development of the shale gas produced from the shale stratum, which is present in a relatively shallow site, and the like, aqueous dispersions in which a polymer is dissolved or dispersed in water have been used recently considering the influence on the environment. Polylactic acid is known as such a polymer.
Specifically, polylactic acid is a substance which exhibits hydrolyzability and enzymatic degradability. Even when polylactic acid is left in the ground, the polylactic acid is degraded by water or enzymes in the ground. Hence, polylactic acid does not exert any adverse influence on the environment. In addition, it can be said that water used as the dispersion medium has almost no influence on the environment, when compared with gasoline or the like.
In addition, when a well is filled with such an aqueous dispersion of polylactic acid, and this aqueous dispersion is pressurized, the polylactic acid penetrates into the vicinity of the well. Then, the polylactic acid is hydrolyzed to lose the shape as a resin, and spaces (i.e., fractures) are formed in the portions into which the polylactic acid has penetrated. Accordingly, the spaces through which the resource flows into the well can be increased.
Further, polylactic acid also functions as a fluid-loss-control agent. Specifically, polylactic acid has a function of inhibiting excessive penetration of water used as the dispersion medium into the ground to minimize the environmental change in the stratum. Since polylactic acid is degraded in the ground, no acid treatment is necessary.
In addition, lactic acid, which is a degradation product of polylactic acid, is an organic acid. After degradation of polylactic acid, lactic acid is released, and this acid erodes shale in the shale stratum. Hence, lactic acid has a function of promoting the pore formation in the shale.
However, polylactic acid has a low hydrolysis rate below 100° C., although it is hydrolyzed relatively rapidly at a temperature of 100° C. or above. Accordingly, when polylactic acid is applied to mining of shale gas produced from a site in the ground where the temperature is low or the like, the efficiency is low, and an improvement has been required.
On the other hand, the use of polyglycolic acid instead of polylactic acid has been proposed. Also, polyglycolic acid is known as a biodegradable resin, and moreover has a higher hydrolyzability than polylactic acid. For example, polyglycolic acid has a much higher hydrolysis rate at a temperature of about 80° C. than polylactic acid. Accordingly, polyglycolic acid is effective as an alternative to polylactic acid.
However, there is such a problem that polyglycolic acid requires much higher costs than polylactic acid. This is a serious disadvantage in the hydraulic fracturing method in which the fracturing fluid is used in a large amount. In addition, under certain temperature conditions, sufficiently satisfactory degradability cannot be obtained.
To efficiently degrade a biodegradable resin, for example, a readily degradable resin composition has been developed whose biodegradability is improved by blending an aliphatic polyester that releases an acid upon hydrolysis (International Publication No. WO2008/038648). In addition, a method for degrading the above-described readily degradable resin composition and the like have been reported (Japanese Patent Application Publication No. 2010-138389). Further, a method for degrading a biodegradable resin in a solution by using any of various hydrolases has been reported (Japanese Patent Application Publication Nos. 2003-284550 and 2005-162832). However, there has been a demand for development of a technology for further improving the degradation rate of a biodegradable resin.